1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) system and, more particularly, to an MRI system employing an image construction technique of obtaining an image having a high S/N (signal-to-noise) ratio by using so-called surface coils.
2. Description of the Related Art
An MRI system requires a relatively long imaging time, e.g., several minutes, for .sup.1 H imaging. In spite of this drawback, an MRI system is considered as an almost perfect system. In .sup.1 H imaging by means of the MRI system, specifically in imaging of a clinicaly stationary portion or a portion which moves slowly, an image having high quality which poses almost no practical problems can be obtained.
Recently, however, a great demand has arisen for high-speed imaging (imaging time: about 50 ms) capable of imaging quickly moving portions (e.g., heart), or imaging of nuclides other than .sup.1 H, e.g., .sup.31 P, .sup.19 F, .sup.13 C, and .sup.23 N. When these nuclides are to be imaged, an increase in S/N ratio is technically an important subject. For example, in high-speed imaging, a short imaging time poses the problem of decreasing the S/N ratio. In imaging of .sup.31 P, the S/N ratio is undesiraby decreased since the amount of .sup.31 P in human body is as very small as about 10.sup.-4 of .sup.1 H.
In a conventional system, in order to increase the S/N ratio, a surface coil is used as a coil for receiving RF signals. The surface coil is brought into contact with a portion of interest of an object to be examined or a body surface near to the portion. The surface coil can detect signals from the portion in contact at a high S/N ratio. However, the drawback of this coil is that only an image near the portion in contact can be obtained. That is, the surface coil cannot perform imaging of an entire predetermined sectional plane of the object at a high S/N ratio. In a conventional method, one surface coil is used, imaging is performed while the position of the coil is sequentially changed, and images obtained at the respective positions are synthesized into an image of a predetermined sectional plane. This method requires complicated adjustment of the system to change the position of the surface coil, and the measuring time will be longer in proportion to an increase in the number of coil positions.
In order to solve such a problem, a technique disclosed in, e.g., U.S. Pat. No. 4,825,162 may be employed. In this technique, a plurality of surface coils are arranged around a desired imaging region of an object to be examined, and MR signals from the object are detected through these surface coils, respectivey. Imaging processing is performed for each of the detected MR signals to obtain image data of different series. Thereafter, pixel data corresponding to the same spatial positions are multiplied by predetermined weighting functions, which are determined in advance on the basis of the distributions of RF fields generated by the respective surface coils, and added, thereby forming data of each pixel. The obtained data are synthesized into an image of the desired region. With this operation, an image having a high S/N ratio can be obtained.
In addition, according to U.S. Pat. No. 4,825,162, since MR signals are simultaneously observed by the plurality of surface coils within the time required to obtain one-region (e.g., one-slice) image data, the technique includes a means for preventing the surface coils from steadily interfering with each other. More specifically, U.S. Pat. No. 4,825,162 discloses a decoupling technique for preventing mutual coupling of the coils. In this technique, the respective surface coils are arranged such that predetermined portions of adjacent surface coils overlap. With this arrangement, even if an RF current having a predetermined frequency flows in one surface coil, no RF current flows in other adjacent surface coils.
In U.S. Pat. No. 4,825,162, however, in order to determine weighting functions in advance, the distributions of RF fields generated by the respective surface coils upon supplying of RF currents thereto must be obtained in advance. In order to obtain the above-mentioned RF field distributions by a computer simulation, the structure, conductivity, and dielectric constant of the object must be obtained. That is, the RF field distributions cannot be easily obtained by a computer simulation. Furthermore, in order to obtain RF field distributions by experiments, homogeneous image data of an entirely desired region to be imaged must be obtained by using, e.g., a homogeneous coil as a transmission coil. This requires an extra period of time.
Furthermore, in U.S. Pat. No. 4,825,162, even if weighting functions are obtained in advance, since weighting/adding processing is performed after Fourier transform of MR signals obtained by the respective surface coils is performed, and a reconstructed image of each surface coil is obtained, Fourier transform processing corresponding to the number of surface coils must be performed.
As described above, in the conventional system, MR signals are simultaneously acquired by using a plurality of surface coils, and an image having a high S/N ratio is obtained by reconstructing images corresponding to the respective surface coils upon Fourier transform of the obtained image data, and performing weighting/adding processing of the reconstructed images. In this system, weighting functions must be determined in advance on the basis of the distributions of RF fields generated by the respective surface coils, and weighting/adding processing is performed after images corresponding to the respective surface coils are reconstructed. For this reason, the conventional system requires an extra processing time and extra processing, and also requires a complex circuit.